The majority of current thermal imagers use semiconducting diode detectors formed from materials such as indium antimonide or lead-tin-telluride. In most of these, an array of one to approximately one hundred of these semiconducting detectors is mechanically scanned to form a complete image. Also, the detectors must be cryogenically cooled (to .perspectiveto. 70.degree. K to 190.degree. K) to perform adequately. In spite of these complexities, this approach is preferred -- at present -- because of the greater sensitivity and higher spatial resolution, compared to alternative approaches.
The major alternative system -- at present -- is the pyroelectric vidicon. This device requires the incident infrared (IR) radiation to be modulated in time to provide the required changes in electronic polarization in the target material, either by chopping the incident radiation or panning the image slowly.
In spite of this drawback the pyroelectric vidicon still holds great promise as a replacement for diode type devices. It is inherently simple, low cost, highly reliable and requires much less power to operate. Except for the mechanical devices used to chop or pan the image, a vidicon system also involves less weight and less initial cost, particularly over those systems which must operate at cryogenic temperatures. The useful sensitivities of current pyroelectric target materials have been extended from the visible or shorter wavelength spectral regions to the region between 8 - 14 micrometers. See R. D. Hudson, Infrared System Engineering, (John Wiley & Sons, New York, 1969); and P. W. Kruse, et al., Elements of Infrared Technology, (John Wiley & Sons, New York, 1962). The above limitations are at present viewed as a materials problem, any or all of which may be improved or eliminated in the future, see for example: L. E. Garn and E. J. Sharp, IEEE Transactions on Parts, etc, Vol. PHP-10, No. 4, Dec. 1974.
The pyroelectric vidicon inherently presents a moving target indication (MTI), however, in most applications it is desirable to have a stationary output image. This has been accomplished by panning the input in an orbital fashion and electronically stabilizing the output image, as shown for example in patent application Ser. No. 663,033, "Electronic Motion Compensation for the Pyroelectric Vidicon," filed Mar. 2, 1976, by John T. Hall. Although this accomplishes the desired end, the advantages of simplicity, low weight, etc. inherent in the pyroelectric approach, are at least partially lost. Attempts to make a competitive system using a chopped input have, so far, been unsuccessful. Using a chopper with a 50% duty cycle should decrease the signal and sensitivity by a factor of 2 (since the system is limited by amplifier noise or electron beam noise, and not by input photon noise). However, experimentally the degradation has been about 2 for medium spatial frequencies and as high as 5 for high spatial frequencies, see: Rex Watton, et al, IEEE Transactions on Electronics Devices, Vol. ED-21, No. 8, Aug. 1974.
One current hypothesis for this observed behavior is that the chopper -- being at some arbitrary temperature, not necessarily near the mean value of the input scene, causes a significant temperature change over the entire pyroelectric surface and consequently acts as a dc offset, decreasing the effective sensitivity. A second significant problem with chopping derives from the fact that the chopping rate -- for practical applications -- is much faster than the reciprocal of the thermal time constant of the pyroelectric surface, thereby causing significant variation in response across the sequentially read surface. This is due to the large variations in integration time between the end of the chopped period and the time when different portions of the surface are read-out; those areas read last will have had time to go through a larger temperature change than those read first, with a corresponding change in output signal level.
The invention is described below as it might be applied to the pyroelectric vidicon, or any parallel input - serial output system. Howeber, it should be noted that the same device -- without the "moving window" feature -- is also applicable to an all parallel systems.